Metal detector

Abstract

A method for reducing unwanted signals, due to a back emf decay, within a receive signal received by a receive coil of a metal detector during a demodulation of the receive signal after a transmission by a transmit coil of the metal detector, including controlling a T/R switch to control a characteristic of the back emf decay to reduce a duration of the period of the back emf decay.

The invention claimed is: 
     
       1. A method for reducing unwanted signals, due to a back-emf decay, within a receive signal received by a receive coil of a metal detector during a demodulation of the receive signal after a transmission by a transmit coil of the metal detector, the method comprising:
 generating a repeating transmit signal cycle, each transmit signal cycle comprising a period of non-zero reactive voltage across the transmit coil, followed by a receive period, wherein the period of non-zero reactive voltage across the transmit coil ends after a high-voltage back-emf period within the period of non-zero reactive voltage across the transmit coil, and wherein the back-emf period ends after a period of the back-emf decay within the back-emf period; 
 having a transmit/receive (T/R) switch, capable of changing between a less conductive state and a more conductive state, for protecting transmit coil receive electronics from a high voltage across the transmit coil during the period of non-zero reactive voltage across the transmit coil, and for allowing the transmit coil receive electronics to receive the receive signal during the receive period; 
 controlling the T/R switch to control a characteristic of the back-emf decay to reduce a duration of the period of the back-emf decay; and 
 controlling at least one characteristic of the period of non-zero reactive voltage across the transmit coil such that a current through the transmit coil and a voltage across the transmit coil approach zero at a same time when the back-emf decay ends to reduce the unwanted signals during the demodulation of the receive signal. 
 
     
     
       2. The method of  claim 1 , wherein the receive period is a period of zero current through the transmit coil. 
     
     
       3. The method of  claim 2 , wherein the transmit coil and the receive coil are the same coil. 
     
     
       4. The method of  claim 1 , wherein the T/R switch changes from the less conductive state to the more conductive state at the commencement of the period of the back-emf decay. 
     
     
       5. The method of  claim 1 , wherein an output of the T/R switch is connected to a resistor whose resistance is selected to produce approximate critical damping when the T/R is in a more conducting state. 
     
     
       6. The method of  claim 1 , wherein an input admittance of the transit-receive switch maybe linear or non-linear. 
     
     
       7. The method of  claim 1 , further comprising:
 having a negative feedback loop which uses, as an input, a signal derived from an output of the T/R switch measured during a period shortly after or immediately following the end of the back-emf decay, to control at least part of the non-zero reactive voltage across the transmit coil. 
 
     
     
       8. The method of  claim 1 , wherein each transmit signal cycle comprises two or more time-separated non-zero reactive periods; and a different negative feedback loop associated with each of the two or more different non-zero reactive periods. 
     
     
       9. The method of  claim 7 , wherein the input to the negative feedback loop includes a demodulation function that demodulates during the receive period, wherein the demodulation function is selected to be approximately balanced to one or both of log-uniform resistive viscous remanent magnetism soil components and uniform conductive half-space soil components. 
     
     
       10. The method of  claim 9 , wherein the demodulation function is balanced to d.c. signals. 
     
     
       11. The method of  claim 1 , wherein the T/R switch is isolated from the transmit coil receive electronics. 
     
     
       12. The method of  claim 1 , further including:
 measuring a current through the transmit coil at the beginning of the receive period in a cycle to control the duration of the period of non-zero reactive voltage in a subsequent cycle such that the current at the beginning of the receive period is substantially zero for the demodulation of the receive signal. 
 
     
     
       13. The method of  claim 1 , further including:
 substantially short-circuiting the transmit coil for a short-circuit period after the period of non-zero reactive voltage before the demodulation of received signal; and 
 measuring a current through the transmit coil after the short-circuit period in a cycle of the repeating transmit signal cycle to control a subsequent cycle such that the current through the transmit coil after the short-circuit period is substantially zero for the demodulation of the receive signal. 
 
     
     
       14. The method of  claim 1 , wherein the step of controlling the transmit-receive switch includes changing the transmit-receive switch from a less-conducting state to the more-conducting state to initiate the back emf decay. 
     
     
       15. The method of  claim 1 , wherein the step of controlling the transmit-receive switch includes changing the transmit-receive switch from a less-conducting state to the more-conducting state during the period of the back emf decay. 
     
     
       16. The method of  claim 1 , wherein an input admittance of the transit-receive switch is selected to control a characteristic of the back emf decay. 
     
     
       17. A metal detector configured to perform the method of  claim 1 . 
     
     
       18. A non-transitory computer readable medium including instructions to perform the method of  claim 1 .